Light rays from a true retroreflector will be directed back toward the source parallel to the original light rays. Several applications involving retroreflectors require that the light leaving the article diverge from the incident angel in order to be effective. For example, light from the hedlamps of an oncoming vehicle, reflected back toward the vehicle by a retroreflective sign, must diverge sufficiently as it leaves the sign to reach the eyes of the driver, who is positioned off-axis from the headlamp beam. In conventional cube-corner retroreflective articles, this cone-like spreading of retroreflected light is obtained through imperfections in the cube-cornerretroreflective elements (e.g. non-flatness of the faces, unintended tilting of the faces from their mutually perpendicular positions, etc.) and through diffraction caused because the retroreflected light exists through an aperture defined by the base edges of the three reflecting faces (see Stamm, U.S. Pat. No. 3,712,706).
However, the spreading of light from a cube corner has many important deficiencies; the cone of the retroreflected light is often too narrow for many uses that require reflected light to be seen farther off-axis; and the three-sided nature of the cube-corner reflective elements gives the retroreflected cone of light an undesirable asymmetric shape. Tanaka, U.S. Pat. No. 3,817,596 increases the divergence or spreading of light rays from the cube-corner retroreflective article by deliberately tilting the faces of the cube-corner retroreflective elements out of perpendicularity or orthogonality.
Although spreading of the light taught in U.S. Pat. No. 3,817,596 increases the observation angles from which the article may be seen by retroreflection, no effort is made to avoid the basic asymmetry that arises from the three-sided nature of a cube-corner retroreflective element.
Heenan, U.S. Pat. No. 3,833,285, changes the divergence or spreading of light from a cube-corner retroreflective article in a different manner, specifically by incorporating into the article a set of special cube-corner retroreflective elements arranged in a row. In each of these special cube-corner retroreflective elements, two of the faces intersect in a line that is aligned along the length of the row of elements and the dihedral angle at the intersection of the two faces is enlarged beyond the conventional 90 degrees, e.g., to 90° 30′ with the result that the light retroreflected by those elements within the row. It is contemplated that in different elements within the row the dihedral angle may be enlarged different amounts so as to spread light into an elongated pattern. The method of construction of the invention in U.S. Pat. No. 3,833,285 is to create a die (see Heenan, U.S. Pat. No. 3,833,285 column 4, line 58 through column 5, line 11) used to impress plastic or metal foil.
Appledorn, U.S. Pat. No. 4,775,219 describes how cube-corner retroreflective articles may be individually tailored so as to distribute light retroreflected by the articles into a desired pattern or divergence profile. The retroreflective article presented in U.S. Pat. No. 4,775,219 carry one one side an array of cube-corner retroreflective elements being formed by three intersecting sets of parallel V-shaped grooves, with at least one of the sets including, in a repeating pattern, a grove side angle hat differs from another groove side angle of the same set. As a consequence of the repeating variations in grove side angle, the array of cube-corner retroreflective elements is divided into repeating sub-arrays that each comprise a plurality of cube-corner retroreflective elements in a plurality of distinctive shapes. At least one of the distinctive shapes is for a non-orthogonal cube-corner retroreflective element, by which is meant that at least one face of the element is tilted at an angle that differs from the angle which would be required for all dihedral angles within the element to be orthogonal, such an element is still regarded as a cube-corner retroreflective element herein, because it closely resembles the ideal cube-corner retroreflective element in shape and function. The overall pattern or the light, the divergence profile for the article, comprises a summation of the different light patterns in which the distinctively shaped cube-corner retroreflective elements in a sub-array retroreflect incident light, and the individual distinctively shaped light patterns can be selected to give the overall pattern a desired shape or contour. The method of construction for the invention described in U.S. Pat. No. 4,775,219 is to scribe a plate (see Appledorn, U.S. Pat. No. 4,775,219 column 10, line 63 through column 11, line 12) that is used as a master for a mould from which the retroreflective article can be cast, embossed or otherwise molded.
Benjamin, W.O. U.S. Pat. No. 0,023,828 describes an invention that provides cube-corner retroreflective articles exhibiting features of rotational symmetry and entrance angularity like that of microsphere-based articles. Such features are achieved by orienting the cube-corner elements randomly so that typically essentially no two cube-corner elements are directly joined and typically no two adjacent elements are in the same orientation. Articles constructed in this manner may have enhanced brightness over microsphere-based articles due to the higher retroreflective efficiency of the cube-corner elements as compared to microspheres.
Rowland, U.S. Pat. No. 3,684,348 for “Retroreflective Material” was to provide a flexible retroreflective shaped material utilizing cube-corners which would conform and adhere to non-planar support surfaces. While this retroreflective material has functioned very well, it has been found that, if stretched during application to a support surface, such as wrapping a tape mask therefrom helically around bicycle handlebars, the cube-corners are distorted. This renders the cube-corner faces non-orthogonal to a degree resulting in significant loss of brightness.
Rowland, U.S. Pat. No. 3,992,080 for “Retroreflective Sheet Material with Controlled Stretch and Method of Making Same” provides a novel stretchable retroreflective sheet material utilizing cube-corner retroreflectors which is easily applied under tension to support surfaces and provides good retroreflection wen stretched a predetermined amount. This is readily attained in an elongated composite retroreflective sheet material with controlled stretch. The composite material comprising a first strip of transparent flexible synthetic resin having a multiplicity of minute cube-corner formations on one surface thereof bonded to a second strip of flexible backing material of lesser length than the first strip when in a relaxed condition with the cube-corner formations disposed adjacent the second strip. The composite retroreflective sheet material is puckered in the relaxed condition with the elimination of the puckered condition providing a visual measurement of the desired degree of elongation of the composite material to avoid distortion of the cube-corner formations.
In summary each modification of the cube-corner pattern requires a new mold, form or embossing element, the construction of which is costly and time consuming. Also, once constructed, the cube-corner article is not adaptable to changing conditions and requirements.
The present invention describes a less costly and time consuming method for obtaining useful cube-corner patterns by distorting cube-corner articles via inelastic stretching such that the cube-corner faces form different permanent angular relationships with each other within a cube-corner retroreflector to produce a desired pattern. Furthermore the cube-corner patterns may be elastically stretched such that the cube corner faces form different temporary angular relationships with each other within a cube-corner retroreflector to produce a temporary desired pattern. Furthermore the article may be heated to reduce the forces necessary for stretching the article. Furthermore the article may be heated without stretching. Heating without stretching has been observed to cause some retroreflective articles to approach true retroreflectors.